A comparator disclosed in JP-A-2004-304515 has an impedance-converting transistor (level-shift transistor) connected to a differential input transistor. In this comparator, the amount of a current flowing through the impedance converter transistor is changed between when an input voltage increases and when the input voltage decreases. Thus, an offset voltage (hysteresis voltage) is provided.
A comparator disclosed in JP-A-2003-179469 has also a level shift transistor. The comparator uses relationship between a collector current and a base-emitter voltage of a bipolar transistor to produce a slight hysteresis voltage. In this comparator, a bias current of the level-shift transistor is changed in accordance with an output of an input differential circuit.
FIGS. 5A and 5B show a circuit diagram of a well-known comparator 1 having a hysteresis characteristic. The comparator 1 includes a differential amplification circuit 5 having transistors Q1-Q4 and a constant current circuit 4, a constant current circuit 6 for outputting a constant current Ia, a transistor Q5, and a resistor R1. The comparator 1 is connected to an output circuit 8 having a resistor R2, a constant current circuit 7, and a transistor Q6. The bases of the transisitor Q1 and the transistor Q2 are used as an inverting input terminal and a non-inverting input terminal of the comparator 1, respectively.
FIG. 6 shows a first threshold voltage VTL and a second threshold voltage VTH of the comparator 1 when a reference voltage Vref is applied to the inverting input terminal of the comparator 1.
When the voltage of the non-inverting input terminal becomes higher than the second threshold voltage VTH, the transistor Q5 is turned off and the transistor Q6 is turned on, as shown in FIG. 5A. An electric current I1 flows from the constant current circuit 6 to the bases of the transistors Q3, Q4 through the resistor R1. The current I1 is given by the following equation:
      I    ⁢                  ⁢    1    =                    R        ⁢                                  ⁢        2                              R          ⁢                                          ⁢          1                +                  R          ⁢                                          ⁢          2                      ×    Ia  
The current I1 changes the threshold voltage of the comparator 1 from the second threshold voltage VTH to the first threshold voltage VTL lower than the reference voltage Vref.
In contrast, when the voltage of the non-inverting input terminal becomes lower than the first threshold voltage VTL, the transistor Q5 is turned on and the transistor Q6 is turned off, as shown in FIG. 5B. An electric current I2 flows from the bases of the transistors Q3, Q4 to the transistor Q5 through the resistor R1. The current I2 is given by the following equation:
      I    ⁢                  ⁢    2    =            VF      -              VCEsat        ⁡                  (                      Q            ⁢                                                  ⁢            5                    )                            R      ⁢                          ⁢      1      
The VF is a P-N junction forward voltage of the transistor Q5. The VCEsat (Q5) is a collector-emitter saturation voltage of the transistor Q5.
The current I2 changes the threshold voltage of the comparator 1 from the first threshold voltage VTL to the second threshold voltage VTH higher than the reference voltage Vref.
Thus, the comparator 1 has a hysteresis voltage VHIS(=VTH−VTL).
For example, the comparator 1 has the hysteresis voltage VHIS of 4.4 millivolts (mV), when the resistor R1 is set to 2 megaohms (MΩ), the constant current Ia is set to 3 microamperes (μA), and temperature is set to 27° C.
When the comparator 1 has a slighter hysteresis voltage VHIS, the current I1, I2 need to be reduced. Therefore, there arises a need to increase size of the resistor such as the resistor R1 in order to increase resistance of the resistor.
In general, the resistor has a chip size of 0.024 mm2 per 1 MΩ. In contrast, a pad of an IC chip of the comparator 1 has a size of 0.01 mm2. Therefore, if the resistance of the resistor is increased, the IC chip size of the comparator 1 is significantly increased so that manufacturing cost of the comparator 1 is increased.